The overall goal of this research project is to isolate and determine the potential of immunocyte precursor cells from the mouse yolk sac. This information will be useful in determining factors which play a role in the ontogeny of the murine immune system. The primary thrust during the initial period of investigation has been to determine the optimum culture conditions. Light and electron microscopy of the yolk sacs following culture has revealed numerous cells similar to stem cells from the bone marrow reported by others. One method of isolating potential stem cells has been to use Ficoll-Hypaque density gradient centrifugation. A Coulter Electronics TPS-1 cell analyser-cell sorter has also been used to isolate potential stem cells. The isolated cells are being analysed for surface markers and mitogenic responsiveness.